In the field of this invention, manufacturers of power amplifiers (PAs), and wireless communication units arranged to use PAs, are constantly aiming to design more efficient topologies capable of providing better performance. In wireless communication standards, the bit rates supported by the wireless communication units continue to increase, thereby creating larger modulation (baseband) signals that can be supported, for example in terms of frequency. As a consequence, larger instantaneous bandwidths (often called Video Bandwidth, (VBW)) are required to be supported in the RF PA of the wireless communication units supporting these wireless communication standards.
The VBW or instantaneous bandwidth performance of an RF PA, relates to a maximum bandwidth of the modulating signal (e.g. corresponding to the bit rate) that the amplifier is capable of amplifying. Next generation wireless standards (such as the long-term evolution (LTE) version of the 3rd generation wireless communication standard termed the 3G Partnership Project (3GPP), WiMAx, 4G standards, etc.) require significantly higher VBW or instantaneous bandwidth performance, of the order of 50-100 MHz when compared to current standards (Global System for Mobile communications (GSM) the General Packet Radio System (GPRS), sometimes referred to as 2.5 G, etc.), which require of the order of 20 MHz. Therefore, the requirement for instantaneous bandwidth is increasing proportionally.
Typically, power amplifier integrated circuits (PAICs) are manufactured by semiconductor device manufacturers, and then purchased by, for example, wireless communication unit manufacturers. The wireless communication unit manufacturers often re-design the input and output matching networks of their transmitter to accommodate the PAICs in their particular transmitter architectures. Consequently, the wireless communication unit manufacturers are able to affect the performance of the RF PA on the PAIC by incorporating particular high value capacitor(s) close to the active device in order to be able to guarantee a given level of performance.
Referring now to FIG. 1, an example of a known RF PA circuit 100 is schematically illustrated. The RF PA circuit 100 comprises an external input circuit 105 that receives a 50 ohm RF input signal 120 and provides an RF input signal to packaged device 110. The packaged device 110 comprises an input matching circuit 125 operably coupling the RF input signal to a single-ended RF PA transistor 130. In this context, the term ‘single-ended’ refers to a common mode amplifier and not a differential mode such as a push-pull amplifier. An output of the drain port of the RF PA transistor 130 is operably coupled to an output matching circuit. The output matching circuit comprises a resonant inductor (Lshunt) 135 in series with a decoupling capacitance 140. The series resonant inductor (Lshunt) 135 and decoupling capacitance 140 are located in parallel across the drain-source ports of the RF PA transistor 130. The decoupling capacitance 140 is selected such that it provides high frequency decoupling of the RF PA transistor output; that is the decoupling capacitance 140 provides a low impedance, for example substantially in a form of a short circuit at radio frequencies, such that the parallel reactance (at RF frequencies) on the drain functions as an inductance. The decoupling capacitance 140 also provides DC blocking of the Vds source, as Vds is applied on the transistor drain.
The packaged device 110 provides an amplified RF output to an external output circuit 115. The external output circuit 115 comprises series inductance 155, an isolation inductor 160 and low frequency capacitive de-coupling 165 (external to the device package). The external output circuit 115 provides a matched RF amplified signal 170. The resonant inductor 135 resonates the transistor's output capacitance 140. As mentioned above, high frequency de-coupling is performed inside the RF transistor device package 110 via capacitor 140.
Capacitor 140 is typically a Si capacitance, for example a metal oxide semiconductor (MOS) or metal/dielectric/semiconductor. This capacitor has a capacitance value that is limited primarily due to the fact that the RF transistor is a large power device, which requires low impedance on the RF transistor's drain, as indicated above. The capacitance value is also limited due to the capacitance density available in existing semiconductor technologies, the need to embed the capacitance as semiconductor devices within the device package and the specific form factor of the transistor used in the device package. In addition, the value of the resonant (shunt) inductor 135, which is created due to the use of wire-bonding in coupling different semiconductor devices (such as RF transistor 130 and capacitor 140), is of the order of 200 pH. Thus, in order for the resonant inductor to cause capacitor 140 to resonate, capacitor 140 located inside the RF transistor device package 110, is typically selected to be of the order of 200 pF, for a 50/80 W transistor, which is suitable for cellular base stations.
In particular, as an alternative to the low frequency capacitive de-coupling 165 (external to the device package), it is also known that such wireless communication unit manufacturers are may introduce one or more high value capacitor(s) 150 close to the active device, where the high value capacitor(s) provides additional low frequency de-coupling to the PAIC. As a consequence, RF PA device manufacturers are required to provide for an integrated circuit (IC) pin 145 on the RF PA device package, located close to the capacitor 140, to enable the wireless communication unit manufacturer to couple the high value capacitor(s) to the PAIC (110), as shown, in order to provide the required additional low frequency de-coupling.
Notably, low frequency decoupling is performed on the printed circuit board, located external to the transistor's PAIC package device 115, such that, in combination, capacitor 140 and external capacitor 150, or external high capacitance value capacitors 165, provide the DC blocking and low frequency decoupling of the RF output.
A significant problem with the typical RF PAIC illustrated in FIG. 1 is the limitation imposed on the VBW performance by the external inductive/capacitive network. The RF PAIC VBW is limited by the resonant frequency of the LC circuit, mostly determined by the 200 pF capacitor 140 and the large isolation inductor 160 and shunt inductor 135. This is illustrated in the low frequency equivalent network circuit on the output side of the amplifier, as shown generally at 175. As also illustrated, the VBW performance is proportional to the inverse square root of the total inductance multiplied by the total capacitance at the RF PA output. At low frequency, the output is high impedance, thereby allowing a voltage to take place and affect the amplifier linearity.
A significantly lower capacitance value is unsuitable, due to the subsequent increase in inductance series loss and the capacitance series loss of using a lower capacitance value, thereby resulting in lower operating efficiency of the RF PAIC. Up until very recently, inherent limitations in capacitor manufacturer technologies prevented a manufacture of capacitor 140 from reaching much higher values, in an acceptable form, due to physical limitations in capacitance density.
It is known that a pure capacitor provides a short circuit as high in frequency as 1/Cw is small at the frequency of operation (w) with respect to a given impedance. Thus, for a pure capacitance, the larger the value the better the short and therefore a pure 1 pF provides a better short circuit at 2 GHz than 200 pF. As mentioned previously, capacitors are in real life resonators, i.e. equivalent to a series RLC. The effect of the series equivalent inductance is to lower the reactive value of that resonator at a given frequency. Below a resonance frequency, given in equation [1], the component acts as a capacitor. Above the resonance frequency, the component acts as an inductance. This is why conventional large value capacitors provide good low frequency short circuits and poor RF short circuits (they act as an inductance, i.e. a high impedance instead of a low impedance). Similarly, small value capacitors with very low parasitic values provide good RF short circuits and poor low frequency short circuits, because of their capacitance value.
For example, existing μF capacitors were known to provide very poor RF short circuits. The equivalent circuit of these capacitors is an R-L-C series network, with a large inductance value. Thus, at RF frequencies of the order of 2 GHz mobile communications, the μF capacitor performed as an equivalent inductance.
US71196113B2 discloses a radio frequency power amplifier device package that comprises such an additional pin on the integrated circuit. The additional pin is necessary on the device package to enable the power amplifier transistor to connect to an external capacitor. Again, the source port of the power amplifier transistor is operably coupled to a first capacitance that is internal to the device package. As explained above, the first capacitance performs two functions: a first function performing direct current (DC) blocking, as Vds is applied on the power amplifier transistor drain; and a second function performing a radio frequency (RF) short circuit at high frequencies, such that the parallel reactance (at RF frequencies) on the power amplifier transistor drain port functions as an inductance. Notably, US71196113B2 requires additional pins to the PAICs used currently in the industry, where high power transistors are provided in two wide lead packages (one lead used as an input and one lead used as an output. Thus, US71196113B2 requires a specific (non conventional) device package having at least one additional lead (ideally two pins) next to the output lead. This leads compatibility problems, mounting problems, etc.
Thus, low frequency decoupling of the power amplifier transistor drain is currently only realisable by locating one or more large value capacitor(s) external to the device package, for example on the application circuit, such as capacitor 150 of FIG. 1.